Melt spun nonwoven webs may be made by a number of processes. The most popular processes are meltblowing and spunbonding, both of which involve melt spinning of thermoplastic material. Meltblowing is a manufacturing process for nonwoven webs in which a molten thermoplastic material is extruded from a row of outlets in a die tip. The streams of thermoplastic material exiting the die tip are immediately contacted with sheets or jets of hot air to attenuate the fibers. The fibers are then deposited onto a collector in a random manner and form a nonwoven web used in such products as diapers, surgical gowns, carpet backings, filters and many other consumer and industrial products.
Generally, meltblown fibers are formed by extruding a low viscosity (i.e., high melt flow rate) thermoplastic material through an array of holes in a meltblown die and impinging the extruded material with high velocity heated air. The resulting fibers have an averaging diameter of between two and five microns. Meltblown fibers are commonly formed from multiple components in which each component may include a unique thermoplastic material having a different chemical composition.
Nonwoven webs of meltblown nanofibers may be made by a process known as electro-spinning that generally involves spinning a solvent-diluted low viscosity polymer in the presence of a directional electric field. Such nonwoven webs, which are characterized by nanofibers of a sub-micron fiber diameter, are known to have utility in a number of applications, such as filtering of particles from fluid streams, for example from air streams and liquid (e.g. non-aqueous and aqueous) streams. In such filtration applications, the interstitial spaces between the nanofibers define small pores that increase the filtration efficiency of the nonwoven. Nonwovens formed from nanofibers also permit the use of lower basis weight, which reduces the cost of products constructed from those nonwovens.
Electro-spinning processes suffer from multiple disadvantages, including the need to remove the solvent from the deposited fibers and an inherently low production rate. Moreover, electro-spinning is not practical on a commercial scale for thermoplastic material since commercially used thermoplastic materials cannot be diluted with a solvent without detrimental consequences to the nonwoven web. The high electric fields required to electro-spin undiluted thermoplastic materials are susceptible to breakdown in air and result in unwanted electrical discharges.
For these reasons, it is desirable to provide apparatus and methods for forming nonwoven webs comprising a majority of meltblown nanofibers that overcome the various problems associated with conventional meltblowing methods for forming such nonwoven webs.